1. Field of the Invention
The present invention is broadly concerned with compositions for dissolving scales (and particularly metal sulfate scales) and methods of using the same. More particularly, the scale dissolver compositions of the invention comprise an aqueous dispersion which includes a salt of a chelating agent, a carbonate, a base, and a component selected from the group consisting of organophosphorus compounds, carboxylic acids, and mixtures thereof. In use, the dissolver compositions are contacted with the target scale, preferably after first heating the composition. The compositions are particularly useful for removing calcium sulfate and barium sulfate scales from gas and oil well equipment, boilers, and pipes.
2. Description of the Prior Art
Many waters contain alkaline earth metal cations such as barium, strontium, calcium, and magnesium, and anions such as sulfate, bicarbonate, carbonate, phosphate, and fluoride. When combinations of these anions and cations are present in concentrations which exceed the solubility product of the various species which may potentially be formed, precipitates form until the respective solubility products are no longer exceeded. For example, when concentrations of barium and sulfate ions exceed the solubility product of barium sulfate, a solid phase of barium sulfate will form as a precipitate. The formation of solubility products are affected by several factors including pH, temperature, pressure, and the presence of a seed crystal. When these solubility products form, they precipitate on the surface of water-carrying or water-containing systems and form adherent deposits or scales which cause serious problems. For example, during gas and oil production, scale plugging of surface and subsurface equipment, tubings, and perforations often lead to severe productivity declines and difficult operating conditions. Of the sulfate scales, barium sulfate is particularly troublesome because of its extreme insolubility.
Previous methods have been attempted to dissolve sulfate scales. For example, U.S. Pat. No. 5,183,112 to Paul et al. discloses a method for dissolving calcium sulfate wherein a scale-removing solvent comprising a chelating agent and the anions of a monocarboxylic acid, oxalates, or thiosulfates is injected into a well followed by sparging of the solvent with air or an inert gas. However, sparging with air or gas is a difficult and messy process, and is quite messy.
Prior art attempts at dissolving barium sulfate include utilizing boiling sulfuric acid. This is impractical and dangerous for dissolving downhole barium sulfate. Another method for removing barium sulfate is to fracture ("frac") it with extremely high pressure. However, this process is quite costly in that it requires special equipment and extra manpower, and results in lengthy well downtimes. Another prior art process for removing barium sulfate is to perforate the scale. This is also not very effective. In some instances, when scales present too much of a problem, the well is simply abandoned and a new well is drilled when scales present too much of a problem. Obviously, this is a very costly solution.